Heretofore, as a process for liquefying and recovering a solvent vapor from a mixed gas, a process for compressing and liquefying the mixed gas, a process for cooling and liquefying the mixed gas and a process for absorbing the mixed gas to a heavy weight hydrocarbon liquid having a low vapor pressure have been known.
However, in the process for compressing and liquefying the mixed gas, the compression pressure must be raised for heightening the recovery efficiency, for that end, a process which comprises providing a saturator before the compressor for the purpose of securing safety and raising the concentration of a solvent vapor in the mixed gas to above the explosion limit, thereafter compressing the mixed gas is adopted. Heightening the concentration of the solvent vapor is evaporating and gasifying the solvent and adding the gasified solvent to the mixed gas, namely, this process carries out an operation opposite to recovery, which is the object of the present invention, therefore, this process does not meet the object of the present invention. In the process for cooling and liquefying the mixed gas, when the mixed gas is cooled to below the freezing point, the moisture of air in the mixed gas coheres, eventually freezes and obstructs the normal driving, which is a drawback. In order to prevent this freezing point phenomenon, sometimes a means of adding a freezing preventing agent or an antifreezing liquid is taken, however, in this case an additional step of separating these additives from a solvent is required, which is not preferable. As a process for absorbing the mixed gas by a heavy weight hydrocarbon liquid having a low vapor pressure, there are (a) a method of heating a solvent, for example, a gasoline liquid to evaporate a light weight component and utilizing the remaining heavy weight component as an absorbing liquid, (b) a method of utilizing kerosene as an absorbing liquid, and (c) a method of utilizing kerosene as an absorbing liquid and using the same by regenerating and circulating. However, in the method (a), not only an apparatus for gathering a non-evaporating component as a heavy weight component is required, but also an apparatus for compressing, cooling and liquefying the evaporated light weight component and returning said component to a storing tank is required, and this method (a) has a drawback in that the apparatus as a whole becomes complicated. In the method (b), although kerosene is available at a low price, not only a large amount of kerosene is required, but also there is a problem in regenerating means of kerosene having absorbed a solvent vapor. In the method (c), a solvent vapor is absorbed by an absorbing liquid such as kerosene or light oil, the absorbing liquid is forwarded to a flush drum, where the absorbing liquid is decompressed by a vacuum pump to evaporate and separate a light weight component (for example, a gasoline component) and the absorbing liquid is regenerated, circulated and reused, however, because the entire solvent vapor is once absorbed by the absorbing liquid and thereafter, all the solvent component is vacuum evaporated and separated by a vacuum pump, devices for cooling and liquefying the evaporated and separated solvent vapor are separately required, which is a drawback.
The present inventors conducted studies with a view to developing a process not particularly requiring a high pressure, an antifreezing liquid and a freezing preventing agent with reference to a process for recovering a solvent vapor free of the drawbacks of these conventional processes, having reached the present invention as a result.